Distillation columns, otherwise known as fractioning or fractionating columns, are used widely for the separation of liquids or vapor-liquid mixtures, for example, in the refining of petroleum or recovery of solvents. Distillation of a liquid essentially works via the application and removal of heat to exploit differences in volatility or boiling points between liquids. Application of heat causes components with lower boiling points and higher volatility to be vaporized, leaving less volatile components (those with higher boiling points) as liquids.
Typically, a “feedstock” that is to be separated is fed into a distillation column at one or more points. Vaporized components exit via one or more vapor take-off points located toward the top end of the distillation column, while the remaining liquid components exit via a condensate removal point, located toward the bottom of the column.
A number of column types have been developed. Two of the most commonly used types are packed bed distillation columns and plate columns, where the column comprises either packing or plates, respectively, to provide a large surface area for vapor/liquid contact within the chamber of the column.
Differences in boiling points cause the more volatile components to transfer from the liquid phase to the vapor phase, and the packing or plates increases the surface area available for phase change to occur, hence increasing the efficiency of separation of the components.
In a packed bed or plate distillation column, the packing material ideally possesses a high surface area to volume ratio. When wetted with the condensed phase, this high surface area develops numerous sites where evaporation may occur within the column. Each site of evaporation generates a dynamic equilibrium between the condensed phase and the vapor phase. The successive generation of these phase changes enriches the vapor phase with a higher proportion of the volatile component. The less volatile condensed phase is slowly removed from these equilibrium sites by gravity.
Taller distillation columns achieve more phase changes with coincident gravitational removal of the less volatile component, to improve separation efficiency of components differing in volatility. However, to achieve this, the size of the column has to be increased. Also, more plates or layers of packing are required to achieve improved separation but at the cost of a slower process, so there must be a trade-off between speed and quality of separation. Taller distillation columns also give rise to large hold up volumes and prolonged equilibrium times. Further disadvantages also experienced with previous packed bed columns include a low separation efficiency for high liquid flow rates and low cost efficiency with low liquid flow rate. Prior packed bed distillation columns also suffer from the possibility of breakage of the packing, both plates and packing material during distillation due to thermal expansion of the packing. In prior plate columns, high pressure drops produced by high throughput can cause flooding and foaming to occur where vapor flows up through liquid.
It is an object of the current disclosure to alleviate at least these known issues.